This operation follows a string of anti-ISIS offensives, such as the campaign to liberate ISIS-held Fallujah in southern Iraq as well as the Kurdish-led assault on the Raqqa province, home to the capital of the so-called Caliphate.

However, many believe that the operation to wrestle Manbij from ISIS control is of greater importance in the battle to “degrade and destroy” ISIS in Syria. The US has long had their eye on Manbij, which is considered a key supply town connecting Raqqa to ISIS-controlled territory close to the Turkish border.

Taking back Manbij from ISIS control would effectively isolate ISIS inside Syria, and crucially would undermine their ability to send foreign fighters and supplies to Raqqa. Although the number of foreign fighters entering Syria has dropped considerably of late, a US military official claimed that up to 500 fighters are still entering every month.

Considering its strategic importance, it appears surprising that it has taken so long for the U.S-led coalition to launch an offensive on Manbij. However, such an operation has been delayed by Turkey’s opposition to the presence of the Kurdish People’s Protection Units (YPG) in SDF forces.

Crossing Turkey’s red lines

Last June, Ankara stated that Kurds crossing the Euphrates was a “red line” for them, thus delaying any Kurdish advance on the town of Manbij. Whilst Turkish opposition has prevented the U.S. from carrying out this operation in the past, for fear of angering their NATO ally, the latest coordination between the U.S. and the SDF in Manbij indicates that Washington is prepared to ignore Ankara’s opposition as they step up their fight against ISIS.

For the Kurds, Manbij also holds significant importance as a takeover of the town will bring them one step closer to their dream of uniting their western canton of Afrin with Kobane. This has been a long-held aspiration for Syrian Kurds, and Manbij will mark a significant step towards making their presence known west of the Euphrates. Abdo Ibrahim, the Afrin Defense minister, claimed that the “SDF will liberate the region and we will connect Afrin and Kobane.”

Throughout the conflict in Syria, Turkey has been staunchly opposed to the YPG, claiming that the YPG is no different from the PKK. Turkey believes them to be a terrorist organisation intent on carving out a Kurdish statelet on their border.

Tensions have been brewing between Ankara and their U.S. allies, reaching boiling point last week, after US Special Forces were photographed with YPG badges on their uniforms while conducting operations north of Raqqa. Ankara was incensed. Mevlüt Çavuşoğlu, the Turkish Foreign Minister, said that the “US should wear Daesh, al-Nusra, and al-Qaeda insignias when they go to other places in Syria and should wear Boko Haram insignias in Africa.”

The SDF, which has had full US support in many operations, is a multi-ethnic force leading the battle against ISIS both in Raqqa and Manbij. Whilst the Kurdish YPG makes up roughly 60 percent of the SDF, it is its Arab counterparts which are leading the operations into the Arab-majority city of Manbij. However, critics say, that whilst there are many Arabs fighting within it, it is the Kurds who hold the power and influence.

It is for this reason that Ankara has so vehemently rejected a SDF-led operation until now. Protecting the northern Aleppo region of Syria has been Ankara’s priority, with Erdoğan stating his long-held desire to create a no-fly zone in the area. Their US allies have continually rejected such a proposal, but have supported Turkish-backed opposition forces in the region.

Turkey has been keen to provide an alternative force to fight ISIS other than the Kurds. Recently, Ankara urged Washington to abandon the YPG, and jointly wipe out the Islamic State with a moderate opposition.

In light of this proposal, Turkish-backed opposition fighters launched an offensive to take back the border town of al-Rai east of Azaz last month. Although initially successful, ISIS launched a counter-attack four days later on April 11, quickly recapturing al-Rai.

Last Friday, ISIS had a further military success against the Turkish-backed rebels, advancing to within 5km of Azaz. The British-based Syrian Observatory for Human Rights claimed this advance was the biggest by ISIS in Aleppo province for two years.

Washington’s dilemma: Turkey or the Kurds?

The performance by Turkish-backed rebels appears to underline the dilemma the US is facing. Despite their insistence that they understand Ankara’s concerns and downplaying of the extent of the disagreement, Washington has staunchly insisted that they do not view the YPG as a terrorist organisation.

Furthermore, tensions between Erdoğan and his Western allies have become increasingly fraught as the strongman president has cracked down on all forms of dissent in his country. On Thursday, Turkey recalled their German ambassador in protest against German MPs overwhelmingly voting in support of a motion describing the massacre of Armenians in 1915 as genocide. Turkish daily newspaper Sözcü ridiculed such a decision with a headline on Friday stating “Hitler’s grandchildren accuse Turkey of having committed a genocide”.

Many believe that Erdoğan’s authoritarian policies have led to growing isolation from Turkey’s traditional NATO allies. The US, who has delicately attempted to balance Turkish concerns alongside their own policies in Syria, may finally be running out of patience.

In stark contrast to Turkish-backed rebels, the YPG, alongside their Arab allies in the SDF, have proven themselves to be the most reliable force in pushing ISIS jihadists back. Last week’s offensive in the Raqqa region underlined this, as did recent successes around Manbij. Latest reports suggest that the SDF forces would reach Manbij within days, after advancing within 10km of the ISIS-held city.

A US official also attempted to calm Turkish opposition. “After they take Manbij, the agreement is the YPG will not be staying,” a US official told Reuters. “So you’ll have Syrian Arabs occupying traditional Syrian Arab land.”

After delaying such close cooperation with the YPG forces for fear of upsetting their NATO ally in Ankara, the US have now thrown all caution to the wind with two large-scale offensives against ISIS in Manbij and Raqqa. These moves will not please Turkey, but with little option after the dismal failure of Turkish-backed rebels to secure the border area between Azaz and Al-Rai, Washington has chosen a policy which prioritises defeating ISIS over political disputes between Turkey and the Syrian Kurds.

If the Manbij offensive proves successful, it may well mark the beginning of a US policy in Syria which shuns Ankara’s opposition in favour of a closer coordination with the YPG. Unless Turkey can come up with an effective answer to this problem militarily, it appears the Syrian Kurds will be the eventual winners in the battle for northern Syria.

* A previous version of this article detailed Sözcü as pro-government.